Silica sols have been used in various fields as abrasives and functional inorganic fillers. The silica sols are stable in the alkaline region, but are instable and easily gelled in the acidic region.
However, there has been a demand for silica sols that are stable at an acidic pH in various fields including acidic abrasives, ceramic fiber materials and chromium surface treating agents.
Meanwhile, alkaline silica sols have another problem. That is, adding an alkaline silica sol to a coating composition which contains an organosilicon compound or a resin as a binder is likely to result in clouding or an increase in viscosity of the coating composition. Films that are obtained from such a coating composition show low transparency.
Thus, there has been a need for silica sols that are stable in the acidic region and can be stably added to a coating composition.
A known approach to improve the stability of a silica sol in the acidic region is to modify the surface of silica particles with aluminum.
For example, Patent Literature 1 discloses a silica sol having a pH of 4 to 5 that is produced by mixing a deionized acidic silica sol with an aqueous solution of acidic aluminum salts, and heating the mixture.
Further, Patent Literature 2 discloses a method for producing silica sols that are stable in the acidic to neutral region, by treating an aqueous silica sol solution having a pH of 6 or more with an ammonia or amine cation exchange resin, adding a metal aluminate, and heat treating the mixture at a temperature of not less than 70° C.
Patent Literature 3 discloses a method for producing silica sols that are stable at pH 5 to 12, by adding an aqueous solution containing a trace amount of aluminum salt to a silica sol.
Patent Literature 4 discloses a method in which an aluminum compound-containing alkaline silica sol is deionized with a cation exchange resin wherein the aluminum compound-containing alkaline silica sol is obtained by adding an acidic silicic acid solution and an aqueous aluminum compound solution to a SiO2-containing aqueous alkali solution or aqueous alkali metal oxide solution, or by adding an acidic silicic acid solution which contains an aluminum compound to a SiO2-containing aqueous alkali solution or aqueous alkali metal hydroxide solution.
Further, Patent Literature 5 discloses a method which includes adding an aqueous alkali aluminate solution to a silica sol which has a particle diameter of colloidal silica of 4 to 30 mμm, a pH of 2 to 9 and an aluminum content in terms of Al2O3/SiO2 molar ratio of 0 to 0.0008 so as to achieve a Al2O3/SiO2 molar ratio in the range of 0.0006 to 0.004; heat treating the mixture at 80 to 250° C. for 0.5 to 20 hours; and bringing the product into contact with a cation exchange resin, or a cation exchange resin and an anion exchange resin to produce an acidic silica sol having a pH of 2 to 5.